The disclosed technology relates to lubricants for internal combustion engine, particularly those fueled with biodiesel fuels.
Biodiesel is a general term for fuel-grade materials derived from natural sources such as vegetable oils. They are often fatty acid methyl esters (“FAME”) such as rapeseed methyl ester (“RME”) of soya methyl ester (“SMA”). Biodiesel fuels are becoming more prevalent for fueling of diesel engines. The increased use of diesel passenger vehicles in Europe and elsewhere is in part a cause of this increase. Current European diesel standard allow for 5% bio-diesel component to be incorporated into fuels, with indications that 10% bio-diesel content will be soon permitted.
Simultaneously, there is continued pressure for reducing particulate matter emissions from diesel engines. Euro 5 requirements, scheduled for implementation in 2009, require reduction in particulate matter to 0.05 g/km. Such levels can only be attained, practically, by use of a diesel particulate filter. These filters require regeneration once they are full of soot, and this is typically achieved by increasing the filter temperature to burn off the soot. The temperature increase is often achieved by post-injection of fuel into the engine cylinder.
However, post-injection of fuel can have the undesirable effect of fuel-dilution of the engine lubricant, as more cylinder wall wetting by the fuel allows more fuel to migrate to and accumulate in the lubricant sump. Bio-diesel components are typically less volatile than conventional mineral diesel fuel, and thus concentration of such components in the sump is exacerbated. In fact, use of bio-diesel fuel (B05, i.e., containing 5% ester) along with post-Injection may result in 40% fuel dilution of the lubricant, and the bio-diesel component may account for 50% of the diluent. These high levels of bio-diesel in the oil may lead to increased oxidation and deposit formation associated with the lubricant.
Detergents based on a variety of metal compounds are known. U.S. Pat. No. 5,688,751, Cleveland et al., Nov. 18, 1997, discloses salicylate salts as lubricants for two cycle engines. Suitable additives include the potassium or sodium salts of C16 alkylphenol and of a C9-18 or C13-18 alkyl salicylate.
U.S. Pat. No. 6,008,165, Shanklin et al., Dec. 28, 1999, discloses a composition for reducing the copper-lead bearing corrosion of a formulation, in particular for engine oils, containing a metal overbased composition comprising at least one carboxylate, phenate, or sulfonate wherein the metal is lithium, sodium, potassium, magnesium or calcium. An example is a sodium overbased sulfonic acid. The composition contains a borated dispersant.
U.S. Pat. No. 6,010,986, Stachew et al., Jul. 31, 1998, discloses a composition for reducing the copper-lead bearing corrosion of a formulation, in particular for engine oils, containing a metal overbased composition comprising at least one carboxylate, phenate, or sulfonate wherein the metal is lithium, sodium, potassium, magnesium or calcium. The composition includes a dispersant that is substantially boron-free.
The disclosed technology provides a lubricant composition suitable for sump lubricated engines fueled by a liquid fuel which includes a bio-diesel component, which exhibits improved oxidation resistance and/or reduced deposit formation in lubricants which contain a portion of the bio-diesel component. This is accomplished by the presence of the alkali metal detergent described hereinafter.